Thermal protection and radio shielding of spark plugs



April M. F. PETERS El" AL 2,197,006

THERMAL rno'rncnon m: mwxo snnswme OF SPARK PLUGS Original Filed Aug. 21} 1935 I 1 I l ,fnvenfors, ME PE TERS Av ASTl/V I BY fitter/la y Patented Apr. 16, 1940 um'rso s'r T s PATENT; OFFICE raorsc'rros AND aanro me or arm ewes.

Melville r. rem, Beltaviiic, an Alien v. Washington, D. c.

eiclaims.

' (Granted under the m oi March a, ma, as amended April 30,1928; 370 0. 6.757)

\ This invention relates to radio shieldedspark plugs of internal combustion engines, and more particularly to constructions to control the transfer of heat from the spark plug to the shielding 5 harness associated with the ignition lead connected to the plug. v 1* It is the general object of this invention to prov vide a shielded spark plug wherein excessive heat 'is conducted away from the sparking points of the plug but the transfer of heat to the associated radio shielding harness in sumcient quantities to damage insulation enclosed in the harness is prevented. In the drawing:

Fig. 1 is a longitudinal section of a center electrode for a spark plug that is very eificient for the purposes of the present invention;

'Fig. 2 is a side elevationof one form of radio shielding for controlling-heat transfer; and

Fig. 3 is a side elevation of another shield for controlling heat transfer.

. This application is a division of our copending application, Serial No. 3'7,17, filed August 21, 1935, now Patent No. 2,186,039, January 9, 1940.

An ignition system for automotive engines comprises a high tension insulated wireconnecting a source of'current at high potential to the insulated center electrode of a spark plug. As increasing voltage is applied to the plug, the gap breaks down and the current passes across the gap between the spark plug points and returns to the source by a path that includes the engine. If the high tension portion of the circuit is surrounded by a metal casing, the greater portion of 36 the current, after passing through the spark gap, returns through the metal casing or shielding hamess. A portion of the electro-magnetic field set up between current in the high tension part of the circuit is neutralized by the electro-mag- 40 netic field of the current flowing in the high tension portion of the circuit. The flow of the major portion of return current through. the shielding harness is due to the fact that this tends to make the total or resultant magnetic field a min-q imum. This is known as the proximity effect. If any portion of the shield circuit does not completely surround the adjacent portion of the high tension circuit, the electro-magnetic field produced by the current flowing in the high tension ignition cable will be distorted by the current returning through the partly surrounding shield so as to tend to produce neutralization of the radiating fields. The shielding harness surrounding the ignition locableisiongincomparismwithtbeshielding around the spark plug. vIf the shieldingaround the spark plug is removed, but means are provided for the current to return through the shield around the cable, the shielding will be effective. Eurthermore, if the path provided for the return current from the spark plug shield-to the cable shielding harness-is placed near the shank of the spark plug, the two fields will be displaced to make the resultant field a minimum.

In view of the foregoing, one of the important aspects of the present invention consists in providing a connection between the shell of the spark plug and the cable shielding harness in which the The relative effectiveness of the described 2 method of suppressing radio interference depends in the general case upon the ratio of the total angle subtended by the conducting strip (or strips) at the axis of the center electrode to the total angle subtended by the openings. The relative 25 effectiveness of the interference suppression depends further on the distribution of the conducting strips around the center electrode. For any given arrangement of the strips there is little change in the effectiveness of shielding as long as 30 the ratio of the angle subtended by the strips to the angle subtended by the openings is equal to or greater than one. Coolingof the plug and cable increases as the openings are increased, so, from this point of view, it is desirable to have the 35 openings as large as possible. An optimum value. from both points of view, would be to have the openings approximately equal to the strips. The

I effectiveness of the suppression of noise increases,

I does not seem'worth while to increase the number of strips to much beyond six.

In certain special arrangements of ignition systems the gains in shielding effectiveness by distributing the conducting strips around the center electrode and decreasing the size of the opening as compared to strips may be quite negligible. This-is true in systems where the bonds between 2 :l I f f i v Q. the engine and the cable shield are criticallyadjusted or where alarge mass of metal (cowling,

etc.), eflectively screensthe'antenna of a radio set from the plugs.

the strips provides adequate shielding.

It is'- important that the sparking points of a spark plug be permitted to attain a sufllcient temperature during operation to prevent the accumulation of oil and other materials that would foul and substantially .the remainder of the electrode- 20 beyond the reduced portidn 6 is bored out as indicated at 1. Around reduced portion Bis a sleeve or sheath 8 of metal having high heat conductivity, such as copper. The length of sleeve 8 is a trifle less than that of the portion 8 when 25 the parts are cold, as indicated by the small gap at 9 to permit sleeve 8 to expand freely'when heated. 4

It is thus apparent that heat will be rapidly conducted away from sparking terminal 5 but so that, upon reaching the diminished cross sectional area of metal around bore I, the flow of heat will be very much retarded and radiation thereof will take place. The metal around the bore 'l'may be of low heat conductive properties, such as 85 stainless steel.

\ The center electrode 4 maybwmounted in a spark plug having firing points "I, as'shown in Fig. 2, carried by the shell I I that is screwed into the enginebodv. This plug is provided with a shielding having spaced longitudinal bars I! for radio shielding purposes, in accordance with the conditions of shielding above set forth. The spaces between the bars are of substantially the same transverse width as the width of the bars 45 themselves. Thebars I! may be, in the portions thereof nearest the engine, of material having good heat' conductive properties and the remainder of each bar be made of material having poor heat conductive properties, or vice versa, depending upon the type of service in which the spark plug is employed. Radiation of the heat is facilitated by the provision of a plurality of trans- In such cases 'a two strip 5 shield with openings 2 or 3' times the width of diminished portion, the length of said sheath'beare-mos versely extending annular fins l3 that expose large radiating areas to air passing thereover.

Fig. 3 depicts another form of our invention wherein the radio shielding is constituted by longitudinally extending bars spaced from each I 5 other by each other, but each bar has formed in it corrugations l5 extending transversely of the bar, whereby the distance traveled by heat flowing along a bar I 4 is very materially increased and the radiating surface is likewise greatly augp mented.

Thelinvention described herein may be manufactured and/or used by or for the Government of 'the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 1

We claim:

1. A center electrode forspark plugs having an axial bore thereinextending inwardly from the outer end thereof for substantially half the length of the electrode and a portion of diminished cross section inwardly of said bore, and a sheath of metal having high thermal conductivi ity disposed around said diminished portion, the normal length of said sheathwhen unheated being somewhat less than the length of said portion to permit free longitudinal expansion of'said' sheath when heated.

2. A center electrode for spark plugs whereof I 4 substantially the outer half has a lower thermal 3o "conductivitythan' the remainder thereof and a portion of undiminlshed cross section extending inwardly therefrom, and a sheath of metal of high thermal conductivity disposed around the said ing somewhat less than the length of said portion to permit free longitudinal expansion of said sheath when heated.

I 3. A radio shielding device for spark plugs, comprising a plurality of spaced apart longitudinally: extendingbars and circumferentially extending fins in heat radiating relation with said bars ad- Jacent at least one end of said bars.

14. A radio shielding device for. spark plugs,

comprising a plurality 'of longitudinally extending spaced apart bars disposable around a spark plug in electrically conducting relation with 'the shell. of said plug, said bars being transversely corrugated, and a portion connecting together the outer ends of all of said bars.

. MELVIILE F. PETERS.

' ALLEN V. ASTIN. 

